misfiring on 1997 Mercedes-Benz S500

replaced no 5 coil and miss cleared up two weeks later no 1 and 6 show misfiring,replaced coils and all new platinum plugs still missing, runs clear when cold and then developes miss within 5 to 10 min, no visible loss of wiring insulation

Check for an air leak in the vacuum hoses and the intake manifold gasket, and the intake piping. Try not to drive it if it is misfiring, you will damage the catalytic converters!The engine module is one of the last items to consider with a problem like this, not impossible but not likely.

Thanks for your guidance, wondering about the wiring harness? I do know that this particular series has had problems in the past with dry rot caused by the trapped heat generated around the wiring harness causing the wiring insulation to get brittle and in some cases shed the insulation, I can find no such issue here and the wires do not feel brittle or stiff. Will check all vacuum hoses ,throttle body gasket and intake manifold for leaks.

hey Carl,The wiring harness can definitely cause this. Make sure that the harness is not pinched under any bolts or brackets. While the engine is running, gently tug on the harness in different spots, see if you can get the engine to misfire. If you see insulation flaking off and bare wire exposed, it is time to replace the harness. Does the engine run rough all the time? Or is it intermittent?

As luck would have it no misfires in the last two days, it has been intermittent and I thought the heating up of the engine may have been triggering it.I willfeel around a little and see if I can find a glitch, I have not found any broken or missing insulation and all the vacuum lines seem to be in pretty good shape. I bought this car in Kansas City and drove it back to Atlanta in June with not a minutes worth of trouble and in very excellent shape for only 86,000 miles. I have four other 87 era 420 sel's that I pittle with so I am competent to do most anything mechanically, unlike a lot of other owners.Again thanks for your help will let you know what else I find if anything.

Question for munkyrench:"Check for an air leak in the vacuum hoses and the intake manifold gasket, and the intake piping. Try not to drive it if it is misfiring, you will damage the catalytic converters!The engine module is one of the last items to consider with a problem like this, not impossible but not likely."

How is the misfire related to a breach in the vacuum system? I had an oil change place improperly reconnect the air intake manifold on my 97 S500 and a few days later I got a cylinder misfire. They oil change shop told me the systems are unrelated. How would I explain the cause of the misfire being the improperly installed vacuum system? Thanks in advance.